Investigation Of Supramolecular Chirality At Air/Water Interface By In-situ Optical Second Harmonic Generation

It is well known that many important physical and chemical processes occur atinterfaces and the research on interface has been an important subject related to physics,chemistry, environment, biology and geology. Air/water interfaces are ubiquitous in thenature and the chirality of the Langmuir film at air/water interface has been studiedextensively in recent years because a variety of chiral control methods such as chiralinduction, memory, transmission and amplification are mainly conducted by the air/waterinterface. Furthermore, chiral films at air/water interface also be served as perfect models ofinvestigating the symmetry breakdown of molecular and supramolecular. Second harmonicgeneration-linear dichroism (SHG-LD) technology have proved to be some advantages ofsingle monolayer sensitivity, in situ detection, interface selectivity and chiral sensitivity andit now become an effective way to study the interface and interfacial chirality.In this thesis presented here the SHG-LD method was used to investigate the chiralstructure of various air/water interfaces and the influence of environment on the chiralstructure. The results are as follows:(1) The basic theory of SHG-LD technology for interface investigations was analyzedand simulated. The chirality SHG response may originate only from the contributions ofelectric dipole transitions or from contributions of both electric dipole transitions andmagnetic dipole transitions. The S-polarization dependence curves of second harmonicintensity for the two cases are simulated in this work. There are four peaks in all simulatedcurves of the S-polarization dependence but the relative peak height of these four peaksdepend on the chiral states of the interfaces. If the interface shows no chirality (DCE=0) allof the four peak values are equal. When the interface shows chirality, there are also fourpeaks in the curves but the four peak values are no longer equal and the relative peakheights are related to the signal of DCE: for the negative DCE four peaks change withincident polarization angle in the following order: low peak, high peak, low peak and highpeak, and vice versa. (2) The distinction of the two cases that only the electric dipole transitions contributesto SHG response and the electric dipole transitions together with magnetic dipoletransitions contribute to it lie in the intensity value of S-polarized at incident polarizationangle of90°. Is(90°)=0imply the contributions of electric dipole transitions need to beconsidered only and Is(90°)>0imply contributions of electric dipole transitions andmagnetic dipole transitions must to be considered. As a result, the second harmonicintensity values of S-polarized at incident polarization angle of90°can be used todistinguish whether the magnetic dipole transitions should be considered. This has animportant significant to the analysis of experimental data when the interfacial chirality isneeded with SHG-LD method.(3) The degree of chiral excess (DCE) which can quantitatively describe the chiralstate of interface are expressed by the achiral components and chiral components ofsecond-order polarizability tensor of electric dipole transitions and magnetic dipoletransitions. The degree of chiral excess can also be expressed as functions of the ratiobetween chiral components and achiral components of second-order polarizability tensor ofelectric dipole transitions when contributions of electric dipole transitions are consideredonly. The signal of DCE is opposite to the ratio but the value of DCE is proportional to theabsolute value of the ratio. Therefore, the degree of chiral excess could not be characteredby the chiral components of second-order polarizability tensor of electric dipole transitionsbut by the ratio between chiral components and achiral components.The degree of chiralexcess can be expressed as complex functions of second-order polarizability tensors ofelectric dipole transitions and magnetic dipole transitions when both of them areconsidered.(4) SHG-LD technology is used to investigate supramolecular chirality formed byachiral porphyrin derivatives at air/water interface. Data fitting and comparative analysis ofall the series of experimental data indicated that supramolecular chirality formed by achiralporphyrin derivatives was already present at the air/water interface and the distributions ofsupramolecular chirality were inhomogenous. The achiral components and chiralcomponents of second-order polarizability tensor of electric dipole transitions and magneticdipole transitions to the interfacial chirality were also inhomogenous. The contributions of electric dipole transitions and magnetic dipole transitions were opposite to each other andthe contributions of magnetic dipole transitions were dominate for positive signal of degreeof chiral excess values while contributions of electric dipole transitions were dominate fornegative signal of degree of chiral excess values.(5) Supramolecular chirality of PARC18at air/aqueous interface was investigated bysecond harmonic generation linear dichroism. It was shown that PARC18formed chiralstructures with stable chiral state at air/water interface. While at air/NaOH solutioninterface, the chiral state changed with time. In addition, on NaOH solution subphase,contributions of magnetic dipole to SHG signals were dominant.